1. Field of the Invention
The present invention relates to stretch wrapping machines with support bearing arrangements.
2. Description of the Related Art
Stretch wrapping machines wrap a web of packaging material around a load by providing relative rotation between a web dispenser and the load. These wrapping machines may be structured in various arrangements to accommodate particular applications. U.S. Pat. Nos. 3,793,798, 4,317,322, 4,302,920, 4,109,445, 4,722,170, 4,858,415, 4,845,920 and 4,866,909 are incorporated herein by reference to illustrate a number of these arrangements.
In U.S. Pat. Nos. 3,793,798, 4,109,445 and 4,722,170, a web dispenser is supported on an arm which extends radially outward and downward from a vertical axis. The arm moves the dispenser in an orbital path about the vertical axis to wrap the load. In U.S. Pat. Nos. 4,845,920 and 4,858,415, the load is placed on a rotatable turntable. As the turntable rotates about a vertical axis, the load is wrapped with a web which is dispensed from a dispenser. In U.S. Pat. No. 4,866,909, the wrapping machine has a rotatable annular frame which carries the web dispenser in an orbital path about a horizontal axis to wrap a load.
In each of these arrangements, either the web dispenser or the load support is a relatively heavy inertial body which travels at substantial velocities. An example of an existing support system for a rotating unit includes a 37 inch diameter slew ring bearing. Such a bearing is constructed from two unhardened low carbon steel rings with machined raceways, and two point contact balls in a single row. The bearing requires continuous lubrication, but has no seals to contain grease or other lubricants. Uneven wear can occur in such a bearing due to unbalanced loading and nonlevel floors. Using a higher precision bearing with ground and hardened raceways and seals could overcome such disadvantages but would also increase the cost significantly. Also, the need for a rigid and highly precise base adds expense.
Another existing support system includes a metal raceway and rollers. The rollers are coated with resilient material such as polyurethane. Such an arrangement has the advantage of being substantially less expensive than a precision bearing. This eliminates the need for special tolerances and lubrication. However, after a period of use, some stretch wrapping machines with this bearing arrangement failed because the polyurethane coating on the rollers separated from the roller surface or otherwise ceased to produce the resilient effect that was necessary for such a bearing.